1. Field of the Invention
This invention relates to a capacitor coupled circuit, and more particularly to a circuit capable of obtaining a stabilized constant output voltage within a short period of time after power supply is switched on.
1. Description of the Prior Art
The conventional capacitor coupled circuit, which is arranged, for example, for a video signal in a video camera or the like, has been arranged to have its time constant set at several ten to several hundred msec for the purpose of permitting passage of a video signal frequency band including a low zone thereof. Such being the arrangement, the conventional capacitor coupled circuit has required a long period of time before the output of the circuit settles down to a normal constant bias voltage FIG. 1 of the accompanying drawings shows an equivalent circuit representing the capacitor coupled circuit. The illustration includes a coupling capacitor C, a signal source impedance Rg, an input impedance Ri of a next stage, a signal source bias voltage V1 and an input bias voltage V2 of a next state.
FIG. 2 shows the transient characteristic of the output voltage V of the above-stated circuit. The illustration includes a point of time t0 at which a power supply is switched on, a point of time t1 at which the level of the power supply rises, and another point of time t2 at which there obtains bias stabilization. A curve between the points of time t1 and t2, representing the stabilizing manner of the above-stated voltage, can be expressed by a formula ##EQU1## As mentioned above, a long period of time, of about several ten to several hundred msec., has been necessary between the points of time t0 and t2 after the power supply is switched on.
Further, in the video camera mentioned above, a pulse clamping circuit which reestablishes the direct-current level of a video signal, is provided in the latter part of the capacitor coupled circuit. However, charging and discharging operations on a clamping capacitor, which serves as a coupling capacitor, must be accomplished within a very short horizontal blanking period, during which clamping pulses are supplied. Therefore, in the event of a momentary voltage change, such as a change that takes place when the power supply is switched on, a period of several msec. has been required before stabilization to a constant voltage is achieved.
FIG. 3 shows an example of arrangement of the above-stated conventional pulse clamping circuit. FIG. 4 shows the transient characteristic of the output voltage V of this pulse clamping circuit relative to clamping pulses P1. In FIG. 3, the elements similar to those shown in FIGS. 1 and 2 are indicated by the same reference symbols. These illustrations include the clamping capacitor C which serves as a coupling capacitor; a clamping switch SW1; a clamping voltage source 15 which is a power source; a resistor Ron which is arranged to be connected to the clamping capacitor C when the clamping switch SW1 turns on; and a clamping voltage Vcp.
Referring to FIG. 4, after, power supply is switched on at a point of time t0, the voltage V rises at a point of time t1 and comes to settle down at a time point t2. A curve representing the stabilization of this voltage is expressed by a formula: V=V1-(V1-V2).multidot.exp(-t/.tau.), (wherein .tau. represents a value .tau.on=(Rg+Ron) C when the clamping switch SW1 is on and another value .tau.off=(Rg+Ri) C when the switch is off. As indicated by the curve, a period of several msec. is required between the time points t0 and t2 before stabilization. This shortcoming of the prior art arrangement is negligible for an apparatus such as a video camera adapted for sensing images over a long period of time. Whereas, in the case of an apparatus adapted for taking a still picture, such as a still picture video camera, an image sensing operation must be accomplished by stabilizing its circuit in a short period of time. Therefore, the above-stated shortcoming has presented serious problem for the apparatuses of that kind.